Laser marking of various materials is very widespread in numerous industries. It is used, for example for marking a serial number, a barcode, a logo etc. . . . The Nd:YAG laser, which is compact and relatively inexpensive, is widely used. Its wavelength of 1064 nm can mark absorbent materials in infrared, such as metals and some plastics or ceramics. However, there exists a range of materials that are transparent at 1064 nm, for which the use of the Nd:YAG laser is impossible. One solution for these materials is to use a frequency multiplier for obtaining radiation at 355 nm, i.e. in ultraviolet. This solution is, however, expensive, cumbersome and unfavourable from the point of view of energy consumption. For this reason, it is preferable to avoid it.
Of the materials that are transparent at 1064 nm, some are liable to transform physically or chemically via the effect of a rise in temperature of the order of grandeur of that produced by laser pulses (typically several tens to several hundred degrees Kelvin). Plexiglas or PMMA (poly methylmethacrylate) is a material of this type. Transparent in the visible range, it has good mechanical properties, such that it is often employed as a replacement for glass, for numerous applications.
Since Plexiglas is slightly absorbent at 1064 nm, several solutions already exist for marking it. Marking at 355 nm, referred to previously, suffers from the aforecited drawbacks. Another method consists in introducing pigments that are photosensitive at 1064 nm into the Plexiglas mass. This solution, however, increases the manufacturing cost of the Plexiglas. Finally, mechanical etching by milling takes a long time and is ill suited to mass marking.
A first alternative to these methods is disclosed in U.S. Pat. No. 5,987,920. This Patent discloses a laser marking method for an article made of a slightly absorbent material using a highly absorbent assisting material of the ceramic type. Said absorbent material is deposited as a sacrificial layer on a support that comes into contact with the article to be etched, or directly on one surface of the article to be etched. Via the effect of the laser illumination, the absorbent material is sprayed and the debris generated are projected against the surface to be marked, which increases the roughness thereof and generates the effect of marking. This method is complex and expensive as it requires depositing a layer of absorbent material on the surface of the article to be marked, then removing it. If a support is used, the lifetime of the support is limited since the sacrificial layer degenerates after several uses. The surface sacrificial layer on the support must be regularly renewed so that the method does not lose efficiency.
U.S. Pat. No. 4,743,463, moreover, discloses a method of marking an article of slightly absorbent material, using a highly absorbent material such as metal. A metal plate, or a sheet of metal is brought into contact with the article to be marked, then illuminated by a laser so as to spray said metal locally, and to redeposit it on the article. Marking is thus obtained by transferring material from the plate to the article. The drawback of this method is the low resistance of the marking to wear and friction.